1. Field of the Invention
The present invention relates to an optical device, and more particularly to an optical device implemented as an optical circulator and a magneto-optical switch.
2. Description of the Related Art
As interest in optical communication increases, the demand for an optical device of high performance, low price, and small size has become greater in recent years, in order to implement optical communications.
FIGS. 1A and 1B are schematic diagrams showing a conventional optical circulator or magneto-optical switch.
The optical circulator or the magneto-optical switch having a configuration similar to that of the optical circulator is implemented by using the non-reciprocal characteristics and inversion of magnetization of a 45-degree Faraday rotator. Basically, it is configured as shown in FIG. 1A.
The optical device shown in FIG. 1A comprises fibers 1006-1 through 1006-4 for inputting/outputting light, lenses 1005-1 through 1005-4 for collimating or focusing light, prisms 1001 and 1004 for splitting/combining light depending on a polarization state, a half-wave plate 1002 and a 45-degree Faraday rotator 1003, both of which are arranged between the prisms 1001 and 1004.
The prisms 1001 and 1004 split input light into two orthogonally-polarized components, and output the parallel beams along respective optical paths. A multilayer interference film is normally used as a means for splitting polarized light. The two prisms 1001 and 1004 are arranged facing each other, and the half-wave plate 1002 for reciprocally rotating a polarization plane by 45 degrees and the 45-degree Faraday rotator 1003 are inserted in between them. If the light proceeds from the fiber 1006-1 to the right in FIG. 1A, the rotation angles of the polarization planes of the polarized components split by the prism 1001 are rotated in the same direction by 45 degrees by both the half-wave plate 1002 and 45-degree Faraday rotator 1003, so that the total rotation angles are 45 degrees plus 45 degrees which equals 90 degrees. On the contrary, if the light proceeds from the optical fiber 1006-4 to the left, the polarization planes are rotated by 45 degrees by the 45-degree Faraday rotator 1003, and counter-rotated by 45 degrees by the half-wave plate 1002, so that the rotation angles are 45 degrees minus 45 degrees which equals 0. Accordingly, light from the fiber 1006-1 proceeds to the fiber 1006-4; light from the fiber 1006-4 proceeds to the fiber 1006-2; light from the fiber 1006-2 proceeds to the fiber 1006-3; and light from the fiber 1006-3 proceeds to the fiber 1006-1. In this way, the capability of the optical circulator can be implemented.
In the meantime, if the electromagnet of the 45-degree Faraday rotator is supplied with an electronic current thereby inverting its magnetization, the Faraday rotation angle is inverted. As a result, the light proceeding from the fiber 1006-4 to the fiber 1006-2, will proceed to the fiber 1006-1, thereby implementing the capability of the magneto-optical switch.
In this configuration, however, cross talk may occur due to the elliptical polarization caused by the 45-degree Faraday rotator, incompleteness of the splitting of the polarized light by the prism, etc. Normally, the amount of cross talk is approximately -25 to -30 dB.
To solve this problem, the optical circulator shown in FIG. 1B was proposed.
In the configuration shown in FIG. 1B, fibers 1010-1 through 1010-4 for inputting and outputting light are arranged, and lenses 1011-1 through 1011-4 collimate or focus the light. Birefringent crystals 1013 and 1014 are arranged so that they face the lenses 1011-1 through 1011-4. In addition, another birefringent crystal 1012 is arranged between the birefringent crystals 1013 and 1014. Half-wave plates 1016-1 through 1016-4 which are arranged in the respective optical paths, and 45-degree Faraday rotators 1015-1 and 1015-2 are inserted between the birefringent crystals 1012 and 1013, 1012 and 1014 respectively.
In FIG. 1B, light input from optical fiber 1010-1 or 1010-2 is split by the birefringent crystal 1013 according to the polarization, and two polarized beams whose polarization planes are orthogonal are polarization-rotated by the respective half-wave plates 1016-1 and 1016-2, so that they will be in the same orientation. After their polarization planes are rotated by the 45-degree Faraday rotator 1015-1, the beams are each refracted in particular directions by the birefringent crystal 1012, and these two polarized beams are again polarization-rotated by the respective half-wave plates 1016-3 and 1016-4, so that they become orthogonal. Then, after their polarization planes are rotated by the 45-degree Faraday rotator 1015-2, they are re-combined into a single beam by the birefringent crystal 1014, and output. Since the polarization direction of light in the central birefringent crystal 1012 differs by 90 degrees in the reverse direction, the light deviates from the optical path in the forward direction and becomes a different beam of light to be output. In this configuration, two non-reciprocal portions are included. Assuming that the cross talk caused by one of the non-reciprocal portions is one hundredth, the cross talk caused by passing through the two non-reciprocal portions will be one ten-thousandth. That is, the cross talk can be reduced significantly.
As described above, cross talk occurs due to the elliptical polarization caused by the 45-degree Faraday rotator and the incompleteness of the splitting of the polarized light by a prism, as in the configuration shown in FIG. 1A. Therefore, the amount of cross talk in FIG. 1B is doubled to approximately -50 to -60 dB, and this amount of cross talk exceeds that of an allowable range for practical use.
However, although the problem of cross talk is solved in the configuration shown in FIG. 1B, the number of components and their total size become large. As a result, the optical device itself becomes expensive.